Agonist-specific voltage-dependent gating of lysosomal two-pore Na+ channels

Endolysosomal channel TMEM175 mediates antitoxin activity of DABMA

DABMA is a chemical molecule optimized from the parent compound ABMA and exhibits broad-spectrum antipathogenic activity by modulating the host's endolysosomal and autophagic pathways. Both DABMA and ABMA inhibit severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a cellular assay, which further expands their anti-pathogen spectrum in vitro. However, their precise mechanism of action has not yet been resolved. TMEM175 is a newly characterized endolysosomal channel which plays an essential role in the homeostasis of endosomes and lysosomes as well as organelle fusion. Here, we show that DABMA increases the endosomal TMEM175 current through organelle patch clamping with an EC50 of 17.9 μm. Depletion of TMEM175 protein significantly decreases the antitoxin activity of DABMA and affects its action on acidic- and Rab7-positive endosomes as well as on endolysosomal trafficking. Thus, TMEM175 is necessary for DABMA's activity and may represent a druggable target for the development of anti-infective drugs. Moreover, DABMA, as an activator of the TMEM175 channel, may be useful for the in-depth characterization of the physiological and pathological roles of this endolysosomal channel.

Descending to inhibit: Antagonist-induced downward shift of VSD II in TPC2

In this issue of Structure, Chi et al.1 report structural and functional studies that reveal the inhibition mechanism of the lysosomal two-pore channel TPC2 by the antagonist SG-094, which is of interest for drug development. Antagonist binding induces the downward displacement of the voltage-sensor domain II (VSD II), which is accompanied by asymmetric conformational rearrangements of the entire channel.

Structural basis for inhibition of the lysosomal two-pore channel TPC2 by a small molecule antagonist

Two pore channels are lysosomal cation channels with crucial roles in tumor angiogenesis and viral release from endosomes. Inhibition of the two-pore channel 2 (TPC2) has emerged as potential therapeutic strategy for the treatment of cancers and viral infections, including Ebola and COVID-19. Here, we demonstrate that antagonist SG-094, a synthetic analog of the Chinese alkaloid medicine tetrandrine with increased potency and reduced toxicity, induces asymmetrical structural changes leading to a single binding pocket at only one intersubunit interface within the asymmetrical dimer. Supported by functional characterization of mutants by Ca2+ imaging and patch clamp experiments, we identify key residues in S1 and S4 involved in compound binding to the voltage sensing domain II. SG-094 arrests IIS4 in a downward shifted state which prevents pore opening via the IIS4/S5 linker, hence resembling gating modifiers of canonical VGICs. These findings may guide the rational development of new therapeutics antagonizing TPC2 activity.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Two-pore channels regulate endomembrane tension to enable remodeling and resolution of phagolysosomes

Phagocytes promptly resolve ingested targets to replenish lysosomes and maintain their responsiveness. The resolution process requires that degradative hydrolases, solute transporters, and proteins involved in lipid traffic are delivered and made active in phagolysosomes. It also involves extensive membrane remodeling. We report that cation channels that localize to phagolysosomes were essential for resolution. Specifically, the conductance of Na+ by two-pore channels (TPCs) and the presence of a Na+ gradient between the phagolysosome lumen and the cytosol were critical for the controlled release of membrane tension that permits deformation of the limiting phagolysosome membrane. In turn, membrane deformation was a necessary step to efficiently transport the cholesterol extracted from cellular targets, permeabilizing them to hydrolases. These results place TPCs as regulators of endomembrane remodeling events that precede target degradation in cases when the target is bound by a cholesterol-containing membrane. The findings may help to explain lipid metabolism dysfunction and autophagic flux impairment reported in TPC KO mice and establish stepwise regulation to the resolution process that begins with lysis of the target.

Convergent activation of Ca2+ permeability in two-pore channel 2 through distinct molecular routes

TPC2 is a pathophysiologically relevant lysosomal ion channel that is activated directly by the phosphoinositide PI(3,5)P2 and indirectly by the calcium ion (Ca2+)-mobilizing molecule NAADP through accessory proteins that associate with the channel. TPC2 toggles between PI(3,5)P2-induced, sodium ion (Na+)-selective and NAADP-induced, Ca2+-permeable states in response to these cues. To address the molecular basis of polymodal gating and ion-selectivity switching, we investigated the mechanism by which NAADP and its synthetic functional agonist, TPC2-A1-N, induced Ca2+ release through TPC2 in human cells. Whereas NAADP required the NAADP-binding proteins JPT2 and LSM12 to evoke endogenous calcium ion signals, TPC2-A1-N did not. Residues in TPC2 that bind to PI(3,5)P2 were required for channel activation by NAADP but not for activation by TPC2-A1-N. The cryptic voltage-sensing region of TPC2 was required for the actions of TPC2-A1-N and PI(3,5)P2 but not for those of NAADP. These data mechanistically distinguish natural and synthetic agonist action at TPC2 despite convergent effects on Ca2+ permeability and delineate a route for pharmacologically correcting impaired NAADP-evoked Ca2+ signals.

Improved approaches to channel capacity estimation discover compromised GPCR signaling in diverse cancer cells

Intracellular signaling orchestrates an organism's development and functioning and underlies various pathologies, such as cancer, when aberrant. A universal cell signaling characteristic is channel capacity - the measure of how much information a given transmitting system can reliably transduce. Here, we describe improved approaches to quantify GPCR signaling channel capacity in single cells, averaged across cell population. We assess the channel capacity based on distribution of residuals by the cellular response amplitude. We further develop means to handle irregularly responding cancer cells using the integral values of their response to different agonist concentrations. These approaches enabled us to analyze, for the first time, channel capacity in single cancer cells. A universal feature emerging for different cancer cell types is a decreased channel capacity of their GPCR signaling. These findings provide experimental validation to the hypothesis that cancer is an information disease, bearing importance for basic cancer biology and anticancer drug discovery.

TPC1 vacuole SV channel gains further shape - voltage priming of calcium-dependent gating

Across phyla, voltage-gated ion channels (VGICs) allow excitability. The vacuolar two-pore channel AtTPC1 from the tiny mustard plant Arabidopsis thaliana has emerged as a paradigm for deciphering the role of voltage and calcium signals in membrane excitation. Among the numerous experimentally determined structures of VGICs, AtTPC1 was the first to be revealed in a closed and resting state, fueling speculation about structural rearrangements during channel activation. Two independent reports on the structure of a partially opened AtTPC1 channel protein have led to working models that offer promising insights into the molecular switches associated with the gating process. We review new structure-function models and also discuss the evolutionary impact of two-pore channels (TPCs) on K⁺ homeostasis and vacuolar excitability.

Expanding the Toolbox: Novel Modulators of Endolysosomal Cation Channels

Functional characterization of endolysosomal ion channels is challenging due to their intracellular location. With recent advances in endolysosomal patch clamp technology, it has become possible to directly measure ion channel currents across endolysosomal membranes. Members of the transient receptor potential (TRP) cation channel family, namely the endolysosomal TRPML channels (TRPML1-3), also called mucolipins, as well as the distantly related two-pore channels (TPCs) have recently been characterized in more detail with endolysosomal patch clamp techniques. However, answers to many physiological questions require work in intact cells or animal models. One major obstacle thereby is that the known endogenous ligands of TRPMLs and TPCs are anionic in nature and thus impermeable for cell membranes. Microinjection, on the other hand, is technically demanding. There is also a risk of losing essential co-factors for channel activation or inhibition in isolated preparations. Therefore, lipophilic, membrane-permeable small-molecule activators and inhibitors for TRPMLs and TPCs are urgently needed. Here, we describe and discuss the currently available small-molecule modulators of TRPMLs and TPCs.

Lysosomal Potassium Channels

Lysosomes are acidic membrane-bound organelles that use hydrolytic enzymes to break down material through pathways such as endocytosis, phagocytosis, mitophagy, and autophagy. To function properly, intralysosomal environments are strictly controlled by a set of integral membrane proteins such as ion channels and transporters. Potassium ion (K⁺) channels are a large and diverse family of membrane proteins that control K⁺ flux across both the plasma membrane and intracellular membranes. In the plasma membrane, they are essential in both excitable and non-excitable cells for the control of membrane potential and cell signaling. However, our understanding of intracellular K⁺ channels is very limited. In this review, we summarize the recent development in studies of K⁺ channels in the lysosome. We focus on their characterization, potential roles in maintaining lysosomal membrane potential and lysosomal function, and pathological implications.

NAADP-Mediated Ca2+ Signalling

The discovery of NAADP-evoked Ca²⁺ release in sea urchin eggs and then as a ubiquitous Ca²⁺ mobilizing messenger has introduced several novel paradigms to our understanding of Ca²⁺ signalling, not least in providing a link between cell stimulation and Ca²⁺ release from lysosomes and other acidic Ca²⁺ storage organelles. In addition, the hallmark concentration-response relationship of NAADP-mediated Ca²⁺ release, shaped by striking activation/desensitization mechanisms, influences its actions as an intracellular messenger. There has been recent progress in our understanding of the molecular mechanisms underlying NAADP-evoked Ca²⁺ release, such as the identification of the endo-lysosomal two-pore channel family of cation channels (TPCs) as their principal target and the identity of NAADP-binding proteins that complex with them. The NAADP/TPC signalling axis has gained recent prominence in pathophysiology for their roles in such disease processes as neurodegeneration, tumorigenesis and cellular viral entry.

PI(3,5)P2 and NAADP: Team players or lone warriors? - New insights into TPC activation modes

NAADP (nicotinic acid adenine dinucleotide phosphate) is a second messenger, releasing Ca²⁺ from acidic calcium stores such as endosomes and lysosomes. PI(3,5)P₂ (phosphatidylinositol 3,5-bisphosphate) is a phospho-inositide, residing on endolysosomal membranes and likewise releasing Ca²⁺ from endosomes and lysosomes. Both compounds have been shown to activate endolysosomal two-pore channels (TPCs) in mammalian cells. However, their effects on ion permeability as demonstrated specifically for TPC2 differ. While PI(3,5)P₂ elicits predominantly Na⁺-selective currents, NAADP increases the Ca²⁺ permeability of the channel. What happens when both compounds are applied simultaneously was unclear until recently.

Lysosomal solute and water transport

Lysosomes mediate hydrolase-catalyzed macromolecule degradation to produce building block catabolites for reuse. Lysosome function requires an osmo-sensing machinery that regulates osmolytes (ions and organic solutes) and water flux. During hypoosmotic stress or when undigested materials accumulate, lysosomes become swollen and hypo-functional. As a membranous organelle filled with cargo macromolecules, catabolites, ions, and hydrolases, the lysosome must have mechanisms that regulate its shape and size while coordinating content exchange. In this review, we discussed the mechanisms that regulate lysosomal fusion and fission as well as swelling and condensation, with a focus on solute and water transport mechanisms across lysosomal membranes. Lysosomal H+, Na+, K+, Ca2+, and Cl- channels and transporters sense trafficking and osmotic cues to regulate both solute flux and membrane trafficking. We also provide perspectives on how lysosomes may adjust the volume of themselves, the cytosol, and the cytoplasm through the control of lysosomal solute and water transport.

COVID-19 metabolism: Mechanisms and therapeutic targets

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) dysregulates antiviral signaling, immune response, and cell metabolism in human body. Viral genome and proteins hijack host metabolic network to support viral biogenesis and propagation. However, the regulatory mechanism of SARS-CoV-2-induced metabolic dysfunction has not been elucidated until recently. Multiomic studies of coronavirus disease 2019 (COVID-19) revealed an intensive interaction between host metabolic regulators and viral proteins. SARS-CoV-2 deregulated cellular metabolism in blood, intestine, liver, pancreas, fat, and immune cells. Host metabolism supported almost every stage of viral lifecycle. Strikingly, viral proteins were found to interact with metabolic enzymes in different cellular compartments. Biochemical and genetic assays also identified key regulatory nodes and metabolic dependencies of viral replication. Of note, cholesterol metabolism, lipid metabolism, and glucose metabolism are broadly involved in viral lifecycle. Here, we summarized the current understanding of the hallmarks of COVID-19 metabolism. SARS-CoV-2 infection remodels host cell metabolism, which in turn modulates viral biogenesis and replication. Remodeling of host metabolism creates metabolic vulnerability of SARS-CoV-2 replication, which could be explored to uncover new therapeutic targets. The efficacy of metabolic inhibitors against COVID-19 is under investigation in several clinical trials. Ultimately, the knowledge of SARS-CoV-2-induced metabolic reprogramming would accelerate drug repurposing or screening to combat the COVID-19 pandemic.

Two-pore channels: going with the flows

In recent years, our understanding of the structure, mechanisms and functions of the endo-lysosomal TPC (two-pore channel) family have grown apace. Gated by the second messengers, NAADP and PI(3,5)P₂, TPCs are an integral part of fundamental signal-transduction pathways, but their array and plasticity of cation conductances (Na⁺, Ca²⁺, H⁺) allow them to variously signal electrically, osmotically or chemically. Their relative tissue- and organelle-selective distribution, together with agonist-selective ion permeabilities provides a rich palette from which extracellular stimuli can choose. TPCs are emerging as mediators of immunity, cancer, metabolism, viral infectivity and neurodegeneration as this short review attests.

TPC2 rescues lysosomal storage in mucolipidosis type IV, Niemann-Pick type C1, and Batten disease

Lysosomes are cell organelles that degrade macromolecules to recycle their components. If lysosomal degradative function is impaired, e.g., due to mutations in lysosomal enzymes or membrane proteins, lysosomal storage diseases (LSDs) can develop. LSDs manifest often with neurodegenerative symptoms, typically starting in early childhood, and going along with a strongly reduced life expectancy and quality of life. We show here that small molecule activation of the Ca²⁺‐permeable endolysosomal two‐pore channel 2 (TPC2) results in an amelioration of cellular phenotypes associated with LSDs such as cholesterol or lipofuscin accumulation, or the formation of abnormal vacuoles seen by electron microscopy. Rescue effects by TPC2 activation, which promotes lysosomal exocytosis and autophagy, were assessed in mucolipidosis type IV (MLIV), Niemann–Pick type C1, and Batten disease patient fibroblasts, and in neurons derived from newly generated isogenic human iPSC models for MLIV and Batten disease. For in vivo proof of concept, we tested TPC2 activation in the MLIV mouse model. In sum, our data suggest that TPC2 is a promising target for the treatment of different types of LSDs, both in vitro and in‐vivo.

Electrophysiology of Endolysosomal Two-Pore Channels: A Current Account

Two-pore channels TPC1 and TPC2 are ubiquitously expressed pathophysiologically relevant proteins that reside on endolysosomal vesicles. Here, we review the electrophysiology of these channels. Direct macroscopic recordings of recombinant TPCs expressed in enlarged lysosomes in mammalian cells or vacuoles in plants and yeast demonstrate gating by the Ca2+-mobilizing messenger NAADP and/or the lipid PI(3,5)P2. TPC currents are regulated by H+, Ca2+, and Mg2+ (luminal and/or cytosolic), as well as protein kinases, and they are impacted by single-nucleotide polymorphisms linked to pigmentation. Bisbenzylisoquinoline alkaloids, flavonoids, and several approved drugs demonstrably block channel activity. Endogenous TPC currents have been recorded from a number of primary cell types and cell lines. Many of the properties of endolysosomal TPCs are recapitulated upon rerouting channels to the cell surface, allowing more facile recording through conventional electrophysiological means. Single-channel analyses have provided high-resolution insight into both monovalent and divalent permeability. The discovery of small-molecule activators of TPC2 that toggle the ion selectivity from a Ca2+-permeable (NAADP-like) state to a Na+-selective (PI(3,5)P2-like) state explains discrepancies in the literature relating to the permeability of TPCs. Identification of binding proteins that confer NAADP-sensitive currents confirm that indirect, remote gating likely underpins the inconsistent observations of channel activation by NAADP.

Parkinson's disease-risk protein TMEM175 is a proton-activated proton channel in lysosomes

Lysosomes require an acidic lumen between pH 4.5 and 5.0 for effective digestion of macromolecules. This pH optimum is maintained by proton influx produced by the V-ATPase and efflux through an unidentified "H⁺ leak" pathway. Here we show that TMEM175, a genetic risk factor for Parkinson's disease (PD), mediates the lysosomal H⁺ leak by acting as a proton-activated, proton-selective channel on the lysosomal membrane (LyPAP). Acidification beyond the normal range potently activated LyPAP to terminate further acidification of lysosomes. An endogenous polyunsaturated fatty acid and synthetic agonists also activated TMEM175 to trigger lysosomal proton release. TMEM175 deficiency caused lysosomal over-acidification, impaired proteolytic activity, and facilitated α-synuclein aggregation in vivo. Mutational and pH normalization analyses indicated that the channel's H⁺ conductance is essential for normal lysosome function. Thus, modulation of LyPAP by cellular cues may dynamically tune the pH optima of endosomes and lysosomes to regulate lysosomal degradation and PD pathology.

The Three Two-Pore Channel Subtypes from Rabbit Exhibit Distinct Sensitivity to Phosphoinositides, Voltage, and Extracytosolic pH

Two pore channels (TPCs) are implicated in vesicle trafficking, virus infection, and autophagy regulation. As Na+- or Ca2+-permeable channels, TPCs have been reported to be activated by NAADP, PI(3,5)P2, and/or high voltage. However, a comparative study on the function and regulation of the three mammalian TPC subtypes is currently lacking. Here, we used the electrophysiological recording of enlarged endolysosome vacuoles, inside-out and outside-out membrane patches to examine the three TPCs of rabbit (Oryctolagus cuniculus, or Oc) heterologously expressed in HEK293 cells. While PI(3,5)P2 evoked Na+ currents with a potency order of OcTPC1 > OcTPC3 > OcTPC2, only OcTPC2 displayed a strict dependence on PI(3,5)P2. Both OcTPC1 and OcTPC3 were activatable by PI3P and OcTPC3 was also activated by additional phosphoinositide species. While OcTPC2 was voltage-independent, OcTPC1 and OcTPC3 showed voltage dependence with OcTPC3 depending on high positive voltages. Finally, while OcTPC2 preferred a luminal pH of 4.6−6.0 in endolysosomes, OcTPC1 was strongly inhibited by extracytosolic pH 5.0 in both voltage-dependent and -independent manners, and OcTPC3 was inhibited by pH 6.0 but potentiated by pH 8.0. Thus, the three OcTPCs form phosphoinositide-activated Na+ channels with different ligand selectivity, voltage dependence, and extracytosolic pH sensitivity, which likely are optimally tuned for function in specific endolysosomal populations.

Endolysosomal cation channels point the way towards precision medicine of cancer and infectious diseases

Infectious diseases and cancer are among the key medical challenges that humankind is facing today. A growing amount of evidence suggests that ion channels in the endolysosomal system play a crucial role in the pathology of both groups of diseases. The development of advanced patch-clamp technologies has allowed us to directly characterize ion fluxes through endolysosomal ion channels in their native environments. Endolysosomes are essential organelles for intracellular transport, digestion and metabolism, and maintenance of homeostasis. The endolysosomal ion channels regulate the function of the endolysosomal system through four basic mechanisms: calcium release, control of membrane potential, pH change, and osmolarity regulation. In this review, we put particular emphasis on the endolysosomal cation channels, including TPC2 and TRPML2, which are particularly important in monocyte function. We discuss existing endogenous and synthetic ligands of these channels and summarize current knowledge of their impact on channel activity and function in different cell types. Moreover, we summarize recent findings on the importance of TPC2 and TRPML2 channels as potential drug targets for the prevention and treatment of the emerging infectious diseases and cancer.

The ins and outs of virus trafficking through acidic Ca2+ stores

Many viruses exploit host-cell Ca2+ signaling processes throughout their life cycle. This is especially relevant for viruses that translocate through the endolysosomal system, where cellular infection is keyed to the microenvironment of these acidic Ca2+ stores and Ca2+-dependent trafficking pathways. As regulators of the endolysosomal ionic milieu and trafficking dynamics, two families of endolysosomal Ca2+-permeable cation channels - two pore channels (TPCs) and transient receptor potential mucolipins (TRPMLs) - have emerged as important host-cell factors in viral entry. Here, we review: (i) current evidence implicating Ca2+ signaling in viral translocation through the endolysosomal system, (ii) the roles of these ion channels in supporting cellular infection by different viruses, and (iii) areas for future research that will help define the potential of TPC and TRPML ligands as progressible antiviral agents.

Lysosomal potassium channels

The lysosome is an important membrane-bound acidic organelle that is regarded as the degradative center as well as multifunctional signaling hub. It digests unwanted macromolecules, damaged organelles, microbes, and other materials derived from endocytosis, autophagy, and phagocytosis. To function properly, the ionic homeostasis and membrane potential of the lysosome are strictly regulated by transporters and ion channels. As the most abundant cation inside the cell, potassium ions (K⁺) are vital for lysosomal membrane potential and lysosomal calcium (Ca²⁺) signaling. However, our understanding about how lysosomal K⁺homeostasis is regulated and what are the functions of K⁺in the lysosome is very limited. Currently, two lysosomal K⁺channels have been identified: large-conductance Ca²⁺-activated K⁺channel (BK) and transmembrane Protein 175 (TMEM175). In this review, we summarize recent development in our understanding of K⁺ homeostasis and K⁺channels in the lysosome. We hope to guide the readers into a more in-depth discussion of lysosomal K⁺ channels in lysosomal physiology and human diseases.

Activation of endo-lysosomal two-pore channels by NAADP and PI(3,5)P2. Five things to know

Two-pore channels are ancient members of the voltage-gated ion channel superfamily that are expressed predominantly on acidic organelles such as endosomes and lysosomes. Here we review recent advances in understanding how TPCs are activated by their ligands and identify five salient features: (1) TPCs are Ca²⁺-permeable non-selective cation channels gated by NAADP. (2) NAADP activation is indirect through associated NAADP receptors. (3) TPCs are also Na⁺-selective channels gated by PI(3,5)P₂. (4) PI(3,5)P₂ activation is direct through a structurally-resolved binding site. (5) TPCs switch their ion selectivity in an agonist-dependent manner.

NAADP: From Discovery to Mechanism

Nicotinic acid adenine dinucleotide 2'-phosphate (NAADP) is a naturally occurring nucleotide that has been shown to be involved in the release of Ca²⁺ from intracellular stores in a wide variety of cell types, tissues and organisms. Current evidence suggests that NAADP may function as a trigger to initiate a Ca²⁺ signal that is then amplified by other Ca²⁺ release mechanisms. A fundamental question that remains unanswered is the identity of the NAADP receptor. Our recent studies have identified HN1L/JPT2 as a high affinity NAADP binding protein that is essential for the modulation of Ca²⁺ channels.

Targeting Two-Pore Channels: Current Progress and Future Challenges

Two-pore channels (TPCs) are cation-permeable channels located on endolysosomal membranes and important mediators of intracellular Ca²⁺ signalling. TPCs are involved in various pathophysiological processes, including cell growth and development, metabolism, and cancer progression. Most studies of TPCs have used TPC^–/– cell or whole-animal models, or Ned-19, an indirect inhibitor. The TPC activation mechanism remains controversial, which has made it difficult to develop selective modulators. Recent studies of TPC structure and their interactomes are aiding the development of direct pharmacological modulators. This process is still in its infancy, but will facilitate future research and TPC targeting for therapeutical purposes. Here, we review the progress of current research into TPCs, including recent insights into their structures, functional roles, mechanisms of activation, and pharmacological modulators.

Two-pore channel (TPC)-mediated endolysosomal Ca²⁺ signalling regulates a variety of processes, including cell proliferation, differentiation, metabolism, viral infection, and cardiac function.

Despite the well-established model that TPCs are Ca²⁺-selective channels indirectly activated by nicotinic acid adenine dinucleotide phosphate (NAADP), it has also been proposed that TPCs as Na⁺ channels are activated directly by phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂].

3D structures of mouse TPC1 and human TPC2 were recently determined, which made it possible for structure-based virtual screening methods to identify pharmacological modulators of TPC.

Recent identification by high-throughput screens of pharmacological modulators that target TPCs will help reveal the molecular mechanisms underlying the role of endolysosomal Ca²⁺ signalling in different pathophysiological processes, and to develop new therapeutics.

Choreographing endo-lysosomal Ca2+ throughout the life of a phagosome

The emergence of endo-lysosomes as ubiquitous Ca²⁺ stores with their unique cohort of channels has resulted in their being implicated in a growing number of processes in an ever-increasing number of cell types. The architectural and regulatory constraints of these acidic Ca²⁺ stores distinguishes them from other larger Ca²⁺ sources such as the ER and influx across the plasma membrane. In view of recent advances in the understanding of the modes of operation, we discuss phagocytosis as a template for how endo-lysosomal Ca²⁺ signals (generated via TPC and TRPML channels) can be integrated in multiple sophisticated ways into biological processes. Phagocytosis illustrates how different endo-lysosomal Ca²⁺ signals drive different phases of a process, and how these can be altered by disease or infection.

Lysosomal Calcium Channels in Autophagy and Cancer

Simple Summary

Autophagy is a cellular self-eating process that uses lysosome, the waste disposal system of the cell, to degrade and recycle intracellular materials to maintain cellular homeostasis. Defects in autophagy are linked to a variety of pathological states, including cancer. Calcium is an important cellular messenger that regulates the survival of all animal cells. Alterations to calcium homoeostasis are associated with cancer. While it has long been considered as cellular recycling center, the lysosome is now widely known as an intracellular calcium store that regulates autophagy and cancer progression by releasing calcium via some ion channels residing in the lysosomal membrane. In this review, we summarize existing mechanisms of autophagy regulation by lysosomal calcium channels and their implications in cancer development. We hope to guide readers toward a more in-depth understanding of the importance of lysosomal calcium channels in cancer, and potentially facilitate the development of new therapeutics for some cancers.

Abstract

Ca²⁺ is pivotal intracellular messenger that coordinates multiple cell functions such as fertilization, growth, differentiation, and viability. Intracellular Ca²⁺ signaling is regulated by both extracellular Ca²⁺ entry and Ca²⁺ release from intracellular stores. Apart from working as the cellular recycling center, the lysosome has been increasingly recognized as a significant intracellular Ca²⁺ store that provides Ca²⁺ to regulate many cellular processes. The lysosome also talks to other organelles by releasing and taking up Ca²⁺. In lysosomal Ca²⁺-dependent processes, autophagy is particularly important, because it has been implicated in many human diseases including cancer. This review will discuss the major components of lysosomal Ca²⁺ stores and their roles in autophagy and human cancer progression.

The role of lysosomal ion channels in lysosome dysfunction

Lysosomes offer a unique arrangement of degradative, exocytic, and signaling capabilities that make their continued function critical to cellular homeostasis. Lysosomes owe their function to the activity of lysosomal ion channels and transporters, which maintain concentration gradients of H⁺, K⁺, Ca²⁺, Na⁺, and Cl^- across the lysosomal membrane. This review examines the contributions of lysosomal ion channels to lysosome function, showing how ion channel function is integral to degradation and autophagy, maintaining lysosomal membrane potential, controlling Ca²⁺ signaling, and facilitating exocytosis. Evidence of lysosome dysfunction in a variety of disease pathologies creates a need to understand how lysosomal ion channels contribute to lysosome dysfunction. For example, the loss of function of the TRPML1 Ca²⁺ lysosome channel in multiple lysosome storage diseases leads to lysosome dysfunction and disease pathogenesis while neurodegenerative diseases are marked by lysosome dysfunction caused by changes in ion channel activity through the TRPML1, TPC, and TMEM175 ion channels. Autoimmune disease is marked by dysregulated autophagy, which is dependent on the function of multiple lysosomal ion channels. Understanding the role of lysosomal ion channel activity in lysosome membrane permeability and NLRP3 inflammasome activation could provide valuable mechanistic insight into NLRP3 inflammasome-mediated diseases. Finally, this review seeks to show that understanding the role of lysosomal ion channels in lysosome dysfunction could give mechanistic insight into the efficacy of certain drug classes, specifically those that target the lysosome, such as cationic amphiphilic drugs.

Human genome diversity data reveal that L564P is the predominant TPC2 variant and a prerequisite for the blond hair associated M484L gain-of-function effect

The endo-lysosomal two-pore channel (TPC2) has been established as an intracellular cation channel of significant physiological and pathophysiological relevance in recent years. For example, TPC2-/- mice show defects in cholesterol degradation, leading to hypercholesterinemia; TPC2 absence also results in mature-onset obesity, and a role in glucagon secretion and diabetes has been proposed. Infections with bacterial toxins or viruses e.g., cholera toxin or Ebola virus result in reduced infectivity rates in the absence of TPC2 or after pharmacological blockage, and TPC2-/- cancer cells lose their ability to migrate and metastasize efficiently. Finally, melanin production is affected by changes in hTPC2 activity, resulting in pigmentation defects and hair color variation. Here, we analyzed several publicly available genome variation data sets and identified multiple variations in the TPC2 protein in distinct human populations. Surprisingly, one variation, L564P, was found to be the predominant TPC2 isoform on a global scale. By applying endo-lysosomal patch-clamp electrophysiology, we found that L564P is a prerequisite for the previously described M484L gain-of-function effect that is associated with blond hair. Additionally, other gain-of-function variants with distinct geographical and ethnic distribution were discovered and functionally characterized. A meta-analysis of genome-wide association studies was performed, finding the polymorphisms to be associated with both distinct and overlapping traits. In sum, we present the first systematic analysis of variations in TPC2. We functionally characterized the most common variations and assessed their association with various disease traits. With TPC2 emerging as a novel drug target for the treatment of various diseases, this study provides valuable insights into ethnic and geographical distribution of TPC2 polymorphisms and their effects on channel activity.

Potential COVID-19 therapeutics from a rare disease: weaponizing lipid dysregulation to combat viral infectivity

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2 has resulted in the death of more than 328,000 persons worldwide in the first 5 months of 2020. Herculean efforts to rapidly design and produce vaccines and other antiviral interventions are ongoing. However, newly evolving viral mutations, the prospect of only temporary immunity, and a long path to regulatory approval pose significant challenges and call for a common, readily available, and inexpensive treatment. Strategic drug repurposing combined with rapid testing of established molecular targets could provide a pause in disease progression. SARS-CoV-2 shares extensive structural and functional conservation with SARS-CoV-1, including engagement of the same host cell receptor (angiotensin-converting enzyme 2) localized in cholesterol-rich microdomains. These lipid-enveloped viruses encounter the endosomal/lysosomal host compartment in a critical step of infection and maturation. Niemann-Pick type C (NP-C) disease is a rare monogenic neurodegenerative disease caused by deficient efflux of lipids from the late endosome/lysosome (LE/L). The NP-C disease-causing gene (NPC1) has been strongly associated with viral infection, both as a filovirus receptor (e.g., Ebola) and through LE/L lipid trafficking. This suggests that NPC1 inhibitors or NP-C disease mimetics could serve as anti-SARS-CoV-2 agents. Fortunately, there are such clinically approved molecules that elicit antiviral activity in preclinical studies, without causing NP-C disease. Inhibition of NPC1 may impair viral SARS-CoV-2 infectivity via several lipid-dependent mechanisms, which disturb the microenvironment optimum for viral infectivity. We suggest that known mechanistic information on NPC1 could be utilized to identify existing and future drugs to treat COVID-19.

Evolutionary Aspects of TRPMLs and TPCs

Transient receptor potential (TRP) or transient receptor potential channels are a highly diverse family of mostly non-selective cation channels. In the mammalian genome, 28 members can be identified, most of them being expressed predominantly in the plasma membrane with the exception of the mucolipins or TRPMLs which are expressed in the endo-lysosomal system. In mammalian organisms, TRPMLs have been associated with a number of critical endo-lysosomal functions such as autophagy, endo-lysosomal fusion/fission and trafficking, lysosomal exocytosis, pH regulation, or lysosomal motility and positioning. The related non-selective two-pore cation channels (TPCs), likewise expressed in endosomes and lysosomes, have also been found to be associated with endo-lysosomal trafficking, autophagy, pH regulation, or lysosomal exocytosis, raising the question why these two channel families have evolved independently. We followed TRP/TRPML channels and TPCs through evolution and describe here in which species TRP/TRPMLs and/or TPCs are found, which functions they have in different species, and how this compares to the functions of mammalian orthologs.